.TH GTT91
.SH NAME
gtt91 \- construction of computer accessible travel time tables
.SH SYNOPSIS
.B gtt91
.SH DESCRIPTION 
.B gtt91
comprises a sequence of routines to set up a representation of
the travel travel time tables for a given velocity model in terms of
the tau-spline procedure of Buland & Chapman (1983), together with
routines for user access (interactive and graphical).
.SS Construction of tau-spline tables:
.br
The two basic stages are:
.br
.I remodl 
\- construction of partial tau integrals for different 
segments of a velocity model of the Earth.
This requires discretisation of the velocity model at
depths corresponding to the slowness employed in the
representation of the tables.  It is at this point in the 
process that the velocity model has to be specified and
linked in with the rest of the routines - subsequently all 
necessary information on the model is carried in the 
direct access table files.  The procedure remodl
generates two table files:
.br
.I     remodl.hed, remodl.tbl
.br
which are used as the input to the second stage (setbrn)
.LP
.I setbrn 
\- construction of the tau-spline coefficients for different
travel time branches corresponding to distinct seismic 
phases. The tau-splines are generated for each of the 
depths at which the model was sampled.  In addition 
auxiliary tables are set up to allow interpolation to 
arbitrary depth.  The procedure setbrn generates two
direct access table files identified by the name of the
velocity model 
.br
e.g.
.I    iasp91.hed, iasp91.tbl
.br
These tables are used in the routine 
.I ttimes91 
which returns
travel times for seismic phases at specified distance 
and in other display and location procedures
.LP
A Makefile template is provided to allow the construction of the
tau-spline tables for different velocity models.  The specification
of the velocity model used for iasp91 is in terms of polynomials
(up to cubic) in normalised radius for different portions of the
velocity model.
The main body of routines used by the gtt91 system are stored in the 
.br
.I $GTT91/libtau.f  
.br
and two include files 
.br
.I $GTT91/limits.inc and $GTT91/ttlim.inc
.br
are needed.
.SS Access to travel time tables:
.br
The simplest mode of access to the travel time tables is interactive 
use of the 
.I ttimes91 
command which allows user specification of 
phases and returns the travel times for all specified phases at
a particular epicentral distance, for a given source depth.
.LP
The travel time information can also be accessed in graphical form
(Postscript output suitable for Laserwriter II) using the 
zstp command which allows windowing in range and time.  
A X-window version (zstx) is under development.
.LP 
The interface to the tau-spline tables is readily adapted
to other computational procedures: 
Printed travel time tables can be generated in a two stage process:
.br
i) Set up a summary file of travel timesfor many different source 
depths and distances using the routine
.br
.I  ttable
.br
which allows interactive specification of source depths
and distances, and includes all available phases.
.br
ii) Run the table sort routine
.br
.I  wtac
.br
which allows the interrogation of the summary file produced by 
.I ttable.  
.br
.I wtac
prompts for a phase code and the number of characters to match and 
also the range of  epicentral distances required.  Output of travel time 
tables is to a specified file in 
.I tbl 
format.  This 
.I tbl 
file can then be
printed using 
.I ptroff 
facilities.
.LP
Other facilities (including information on take-off angles
for source mechanism studies) are under development.        
.SH ENVIRONMENT
.IP GTT91
Home directory for sources of gtt91 routines 
access required for constructing modified
travel time tables
.SH SEE ALSO
ttimes91, shake91, zstp91, zstx91, syax91
.SH AUTHORS
R. Buland,
National Earthquake Information Centre, U.S. Geological Survey
Golden, Colorado
.br
B.L.N. Kennett,
Research School of Earth Sciences, Australian National University
Canberra, Australia 
